Many modern wireless devices rely on radio communication, which includes the reception and transmission of Radio Frequency (RF) signals. The transmission of such RF signals typically requires the use of an RF power amplifier (PA), and RF PAs are often included within wireless devices. Such devices include terminal devices and radio base stations within wireless mobile telecommunications systems, e.g., systems based upon standards from the 3rd Generation Partnership Project (3GPP), and which are commonly known as 3G, 4G, and 5G systems. The upcoming 5G system relies heavily on multiple-input multiple-output (MIMO) antenna technology, which uses multiple antennas and RF chains on both the transmit and receive side of a communication link. The multiple RF transmit chains used within a MIMO wireless device, e.g., a base station within a 5G system, requires an increased number of RF PAs relative to prior (non-MIMO) systems.
RF PAs require very good linearity so as to maximize the data throughput in a radio transmission. RF PAs are typically comprised of one or more power transistors, which must be operated within a particular region (e.g., their linear region) to provide such good linearity. This, in turn, requires that the transistor(s) within an RF PA be appropriately biased. For example, a field-effect transistor (FET) within an RF PA may have a constant bias voltage applied to its gate (control terminal) so that the default operation of the FET, with no RF input signal, is at a midpoint of its linear operating region. This type of biasing is typical for a class-A amplifier and leads to an amplified output that linearly follows a (small-signal) RF signal input to the amplifier for the largest possible range of the input RF signal. Other amplifier types, including, e.g., Doherty PAs, may not use a bias that places a FET at a mid-point of the FET's linear region, but nonetheless require an appropriate and consistent bias. The fundamental objective in the generation of an RF PA bias is to maintain the quiescent current through any RF power transistor(s) of the RF PA at a desired level that assures the RF PA operates within a desired class of operation. Given that various system parameters (e.g., temperature, long-term drift of the transistor conductivity relative to its control voltage) change during operation or during the lifetime of an RF PA, a fixed bias may be inadequate to achieve the linearity required by an RF PA.
Techniques for determining an optimal RF PA bias across a range of conditions are required. These techniques should use minimal resources, such that they may be feasibly applied to a large number of RF PAs, as may be included within a wireless transmitter of a MIMO system.